Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, lap-top computers and even electric cars, efforts have increasingly been made towards the research and development of batteries. In this aspect, electrochemical cells have attracted the most attention. The development of rechargeable secondary batteries has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Among currently available secondary batteries, lithium ion secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than conventional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, and the like.
Generally, a secondary battery includes a cathode, an anode, a separator interposed between the cathode and the anode, and an electrolyte solution, and among them, a separator is an important factor for determining a life span of a secondary battery and serves to electrically insulate the cathode from the anode. To use as a separator, it is preferred to have high ion permeability and excellent mechanical strength, as well as long-term stability against an electrolyte solution of an electrochemical cell.
Studies have been conducted to use, as a separator, by processing polyolefin-based materials or heat resistant materials among separator materials in a form of a film or a non-woven fabric, and a non-woven fabric has a limitation on its use due to a pore having a relatively large diameter, despite an advantage of a low cost.